1. Field of the invention
The present invention relates to a valve nozzle used for a molding machine for synthetic resin, and in particular relates to a valve nozzle and a shutoff nozzle used for hot liner molding.
2. Description of the prior art Japanese laid-open patent publication No; 163542 of 1982 discloses a valve nozzle which is to open and close a flow path by causing the tip end of a shutoff pin to be brought into contact with the gate at the tip end part of the nozzle head, wherein the shutoff pin is pressed into the gate direction by a coil spring incorporated in an enclosed chamber, and if the pressure of the resin is increased over a certain figure and overcomes the pressing force of the coil spring, the shutoff pin moves in the direction of the enclosed chamber to cause the gate to be opened.
Furthermore, Japanese laid-open patent publication No. 290021 of 1986 describes a valve nozzle constructed so that a ring having a circular hole is disposed in the interior space of the nozzle head, a flow path is closed by causing the tip end of a spool to be engaged in the circular hole, and this state is maintained by a pressing force of a coil spring engaged with the spool in the opposite gate direction, wherein the pressure of the resin is increased over a certain figure and overcomes the pressing force of the coil spring, the spool moves in the gate direction to cause a flow path to be formed between the circular hole of the ring and a groove prepared at a part of the head of the spool.
Such a type as described in the abovementioned Japanese laid-open patent publication No. 163542 of 1982 has a difficulty by which molded products may be liable to be broken in the vicinity of the gate due to a great residual stress at the point when a resin path is closed. Furthermore, when the resin is extruded through the gate with the resin path open, the resin moves straight along the space of the gate and advances, changing its flow direction to the cross direction when it is brought into contact with a metal mold. Therefore, in this construction, the flow of resin around the gate may be worsened.